Glaucoma is a major cause of blindness worldwide. The blindness that results from glaucoma can involve both central and peripheral vision and can have a major impact on an individual's ability to lead an independent and productive life. Pathophysiologically, glaucoma is an optic neuropathy (a disorder of the optic nerve) observed most typically in the setting of an elevated intraocular pressure. Dramatic and/or prolonged increases in intraocular pressure cause changes in the appearance (“cupping” or “excavation”) and function (“scotomas” or “blind spots” in the visual field) of the optic nerve. If the pressure remains high enough for a long enough period of time, total, irreversible vision loss occurs. High intraocular pressure results from an imbalance in intraocular fluid production versus outflow. Glaucoma surgeries, also referred to as filtering procedures, are designed to improve intraocular fluid balance by augmenting fluid outflow, thereby lowering intraocular pressure.
The most commonly performed filtering procedure for treating an eye with an elevated intraocular pressure is a trabeculectomy. During a trabeculectomy, the conjunctiva or the transparent tissue that covers the sclera is incised and reflected to expose the sclera at and immediately posterior to the corneoslceral junction or limbus. A partial thickness scleral flap is then made and dissected forward into the cornea. The anterior chamber is entered beneath the scleral flap by excising a section of deep sclera and together with the underlying trabecular meshwork. The scleral flap then sutured back into place so as to regulate flow of intraocular fluid to the subconjunctival space. The potential subconjunctival space surrounding and posterior to the scleral flap is then dissected so as to facilitate subconjunctival flow of fluid prior to suturing the conjunctival incision tightly closed. Post-operatively, the aqueous fluid passes through the sclerectomy/trabeculectomy, beneath the scleral flap, and into the expanded subconjunctival space. The fluid then is either absorbed through blood vessels in the conjunctiva or traverses across the conjunctiva to mix with the tear film.
Trabeculectomy is associated with many problems. Fibroblasts that are present in the subconjunctival space are induced by the trauma of surgery to proliferate and migrate, thus promoting scar formation and closure of the subconjunctival outflow path. Of eyes that have an initially successful trabeculectomy, eighty percent will fail from scarring within three to five years after surgery. Failure from scarring is particularly common in children and young adults. To minimize fibrosis, surgeons often apply antifibrotic agents, such as mitomycin C (MMC) and 5-fluorouracil (5-FU), to the scleral flap and surrounding subconjunctival space at the time of surgery. The use of these agents has increased the success rate of trabeculectomy, but also has increased the prevalence of overfiltration and hypotony, or excessively low intraocular pressure (usually less than 6.0 mmHg). Prolonged hypotony is associated with retinal distortion, loss of vision, and ultimately, loss of structural integrity of the eye,
Trabeculectomy creates a pathway for intraocular fluid to escape the eye. At the same time, however, trabeculectomy creates a potential pathway for bacteria that normally colonize the surface of the eye and eyelids to enter the eye—causing a severe intraocular infection known as endophthalmitis. In addition to subconjunctival scarring, hypotony and endophthalmitis, there are other recognized complications of trabeculectomy. The conjunctiva over the sclerectomy/trabeculectomy site, know as a “bleb,” can tear, producing excessive filtration and profound hypotony. In addition, large anteriorly located blebs can be irritating, can disrupt the normal tear film, leading to blurred vision, and can make it difficult for patients to wear contact lenses. All of the complications of trabeculectomy stem from poorly directed flow intraocular fluid once it enters the subconjunctival space.
Many glaucoma drainage devices have been used in the management of resistant glaucoma as an alternative to trabeculectomy. These include use of shunting devices, which include the Molteno device, the Ahmed device, the Kruppin device and the Baerveldt device. Other shunting devices include the Schocket implant, the Ex-Press R50, and the AGV silicon tube. A publication entitled, Glaucoma Drainage Devices: A Systematic Literature Review and Current Controversies provides a systematic review of the literature and outlines the current issues involving different glaucoma drainage devices and their design, overall surgical success and complications following glaucoma drainage device insertion (e.g., see Hong, et al, A Systematic Literature Review and Current Controversies; Survey of Ophthalmology; Volume 50, Number 1, January-February 2005).
These known devices suffer from certain shortcomings. One problem with known glaucoma drainage devices is that the location of their insertion can cause damage to the anterior chamber structures, such as the cornea, the iris, or the intraocular lens. Another problem with such devices is related to complications caused by their occlusion by fibrosis and/or infection. Another problem with the know devices is their associated difficulty is establishing and regulating a sufficient amount of flow that would allow intraocular pressure to be titrated following the procedure. Another known problem of such devices is the problems associated with double vision, limitation of eye movement and so on that can be encountered with shunting devices with external reservoirs.
There is therefore a need for a device that can be used in combination with trabeculectomy or other intraocular drainage devices that can optimize fluid flow from inside the eye to the subconjunctival space and, in some embodiments, posteriorly to the retrobulbar space.